For example, a Francis-type turbine and a pump turbine are known as a hydraulic machine. FIG. 15 is a plan view showing a stationary blade row channel of a generally Francis-type pump turbine, which is formed by guide vanes 120 and stay vanes 130. The guide vanes 120 are circumferentially arranged with an interval there between to surround a runner, not shown, radially outside the runner. The stay vanes 130 are circumferentially arranged with an interval there between, radially outside a blade row of the guide vanes 120. A not-shown casing is disposed radially outside the stay vanes 130.
White arrows in FIG. 15 depict an orientation of water flow during a turbine operation, while black arrows therein depict an orientation of water flow during a pump operation. As shown by the white arrows, in the Francis-type pump turbine, during the turbine operation, water from the casing flows through the stay vanes 130 and the guide vanes 120 into the runner. The runner converts water energy to a torque, so that a generator motor is driven through a main shaft, not shown. The water having exited the runner is introduced to a tailrace through a suction pipe, not shown. On the other hand, during the pump operation, water flows reversely to the turbine operation, as shown by the black arrows. Namely, water from the suction pipe passes through the runner to flow through the guide vanes 120 and the stay vanes 130, and flows out from the casing to an upper reservoir.
FIG. 16 is a view of the guide vane 120 seen along the circumferential direction. FIG. 17 is a sectional view of the guide vane 120 taken along the A-A line in FIG. 16. The guide vane 120 in such a Francis-type pump turbine is rotatable about a guide vane rotation shaft 121. By rotating the guide vane 120 to vary an angle thereof, a channel area of a channel formed between the adjacent guide vanes 120 can be varied. Thus, a power generation output can be adjusted by varying an amount of water to the runner.
As shown in FIG. 16, in the Francis-type pump turbine of this kind, the guide vane 120 is located between an upper cover 111, which defines a part of the channel running from the casing up to the runner and is located on the side of the generator motor, and a lower cover 112, which is spaced apart from the upper cover 111 and is positioned on the side of the suction pipe. As described above, since the guide vane 120 is rotatable for adjusting an amount of water, it is necessary to provide a gap (g) between the guide vane 120 and the upper cover 111, and between the guide vane 120 and the lower cover 112, in order to avoid contact there between.
However, such a gap (g) poses a problem in that it increases a hydraulic power loss. How water flows through the gap (g) is described with reference to FIGS. 16 and 17. FIG. 17 schematically shows water flows (a) to (c) by arrows, based on an analysis result.
Namely, as shown in FIGS. 16 and 17, due to the provision of the gap (g), a gap flow (b) passing through the gap (g) is generated, apart from a main flow (a) flowing along a blade surface of the guide vane 120. There is a possibility that the gap flow (b) comes together with the main flow (a) flowing between the adjacent guide vanes 120, which in turn generates a turbulent flow (c) (see FIG. 17) in the vicinity of a rear end of the guide vane 120 above and below thereof. Thus, a separate flow that does not flow along the guide vane 120 is generated, as a result of which a hydraulic power loss disadvantageously may increase. In addition, since the turbulent flow (c) near the rear end of the guide vane 120 becomes a flow flowing in a direction different from that of the main flow (a) and flows into the runner, there is a possibility that a flow at an runner inlet becomes a turbulent flow which increases a runner loss.
In order to reduce the aforementioned gap flow (b), it can be considered that the gap (g) is made smaller. However, the smaller a distance between the guide vane 120 and the upper cover 111/the lower cover 112 is, the greater the risk of interference between the guide vane 120 and the upper cover 111/the lower cover 112 becomes, during the rotation of the guide vane 120. In addition, if a foreign matter such as a stone enters, there is a greater risk that the stone is caught by the gap (g). Thus, reduction of the gap (g) in size has limitations.
In order to reduce the gap flow in a hydraulic machine, a technique for providing a groove in an end face of a guide vane is known. According to this technique, since an area of the gap is largely increased by the groove, a pressure inside the groove locally varies. Thus, a flow velocity of the flow flowing into the groove lowers, whereby a leakage prevention effect can be obtained. In addition, there is known another technique for providing a recessed groove in a cover wall surface facing an end face of the guide vane.